In small electronic appliances such as note-type personal computers, smal (phones, mobile phones, etc., which have been provided with increasingly higher performance and more functions, electronic devices such as microprocessors, imaging chips, memories, etc. should be mounted densely. Accordingly, to prevent malfunction due to heat, the dissipation of heat generated from such electronic devices has become increasingly important.
As a heat-dissipating sheet composed of flaky carbon such as graphite for electronic devices, a graphite sheet obtained by heat-treating polyimide at 3000° C. in an oxygen-free atmosphere to remove hydrogen, oxygen and nitrogen, and annealing the remaining carbon for crystallization, is used. The graphite sheet has as high thermal conductivity as 800 W/mK in an in-plane direction and 15 W/mK in a thickness direction. However, because expensive polyimide is heat-treated at a high temperature, the graphite sheet is extremely expensive.
As an inexpensive heat-dissipating sheet of flaky carbon, JP 2006-306068 A discloses a heat-conductive sheet comprising at least a graphite film and an adhesive resin composition, which is a reaction-curable vinyl polymer. This graphite film is (a) expanded graphite formed by an expanding method, or (b) obtained by heat-treating a polyimide film, etc., at a temperature of 2400° C. or higher. The expanded graphite film is obtained by immersing graphite in acid such as sulfuric acid, etc. to form a graphite interlayer compound, heat-treating the graphite interlayer compound to foam it, thereby separating graphite layers, washing the resultant graphite powder to remove acid, and rolling the resultant thin-film graphite powder. However, the expanded graphite film has insufficient strength. Also, the graphite film obtained by the heat treatment of a polyimide film, etc. is disadvantageously expensive despite high heat dissipation.
JP 2012-211259 A discloses a heat-conductive sheet comprising graphite pieces, which comprise pluralities of first graphite pieces obtained by thinly cutting a thermally decomposed graphite sheet, and second graphite pieces smaller than the widths of the first graphite pieces, at least the first graphite pieces connecting both surfaces of the heat-conductive sheet. This heat-conductive sheet is obtained, for example, by blending the first and second graphite pieces with a mixture of an acrylic polymer and a solvent, and extruding the resultant blend. However, the extruded heat-conductive sheet does not have sufficient heat dissipation, because of a high volume fraction of the resin.
JP 2006-86271 A discloses a heat-dissipating sheet as thick as 50-150 μm comprising graphite bonded by a binder resin having a glass transition temperature of −50° C. to +50° C., such as an amorphous saturated copolyester, a mass ratio of graphite/binder resin being 66.7/33.3 to 95/5. This heat-dissipating sheet is produced by applying a slurry of graphite and a binder resin in an organic solvent to a parting-agent-coated film on the side of a parting layer, drying the slurry by hot air to remove the organic solvent, and then pressing it, for example, at 30 kg/cm2. JP 2006-86271 A describes that the pressing of a graphite/binder resin sheet improves its thermal conductivity. Though a slurry of graphite, a binder resin and an organic solvent is coated by one step in JP 2006-86271 A, it has been found that one-step coating provides a non-uniform graphite distribution. In addition, because a mass ratio of graphite to a binder resin is not so high in Examples (80/20 in Example 1, and 89/11 in Example 2), inherently high thermal conductivity of graphite cannot be fully used.
JP 11-1621 A discloses a high-thermal-conductivity, solid composite material for a heat dissipater comprising highly oriented graphite flakes and a binder polymer polymerized under pressure. This solid composite material is produced by mixing graphite flakes with a thermosetting monomer such as an epoxy resin to prepare a composition comprising at least 40% by volume of graphite, and polymerizing the monomer while compressing the composition under sufficient pressure to align graphite substantially in parallel. JP 11-1621 A describes that the volume fraction of graphite in the composite material can be 40-95%, and is preferably 55-85%. However, graphite flakes are unevenly distributed in an epoxy resin containing graphite flakes at as high a concentration as 95%. Accordingly, JP 11-1621 A describes only experimental results when the volume fraction of graphite flakes is 60%.
As described above, conventional heat-dissipating sheets containing graphite blended with binder resins cannot sufficiently use high thermal conductivity of graphite because of low thermal conductivity of binder resins.
In addition, when the distribution of graphite is non-uniform, the heat-dissipating film exhibits further reduced heat dissipation, and provides non-uniform graphite distribution in a heat-dissipating film cut to a predetermined shape and size for being disposed in a small electronic appliance, resulting in unevenness performance.